The field of application of the invention is more particularly that of starter generators for gas turbine aeronautical propulsion engines mounted on aircraft. The invention may be applied, however, to other types of turbomachines, for example industrial turbines, helicopter turbines or auxiliary power unit (APU) turbines.
Such a starter generator comprises a rotary electric machine that is intended to be mechanically coupled to a shaft of a turbomachine. The starter generator is capable of operating in a generator mode, in what is termed a generation phase, during which the turbomachine rotates the shaft and the rotary machine transforms the mechanical energy of rotation of the shaft into electrical energy that is intended to supply a secondary network, for example an onboard network of an aircraft, with electrical power. The starter generator is also capable of operating in starter mode, during what is termed a startup phase, during which the rotary electric machine supplies mechanical power to the shaft of the turbomachine in order to set and to drive the shaft of the turbomachine in rotation so as to start up the turbomachine.
The rotary machine of such a starter generator, or S/G, typically comprises a main electric machine, an exciter and optionally an auxiliary generator. These elements of the rotary machine are mounted on a common shaft that is mechanically coupled to a shaft of the turbomachine. Such a starter generator is a brushless starter generator.
The main electric machine forms a main electric generator (or alternator) operating in synchronous mode. The main electric machine possesses a rotor winding and stator windings which, when it is operating in synchronous generator mode, convert mechanical energy supplied by a shaft that is mechanically coupled to the turbomachine into AC three-phase electrical energy supplying an onboard network of an aircraft with power via a power supply line. For aeronautical applications, the AC onboard network of aircraft, supplied with power via the voltage delivered by the starter generator operating in generator mode, consists of three phases with 115 V RMS from phase to neutral and 200 V RMS between phases. The frequency thereof may be fixed at 400 Hz or variable (generally between 350 and 800 Hz).
The exciter comprises a stator comprising two stator windings, one of which is supplied with DC current during the generation phase and the other of which is supplied with AC current during the startup phase. These respective windings are referred to as the DC stator winding and AC stator winding throughout the rest of the text. The AC voltage delivered by the main generator in the generation phase is regulated by means of an alternator regulator or GCU (generator control unit) which supplies the DC stator winding of the exciter with a DC current, referred to as the regulating current, during the generation phase and which does not supply it with current in the startup phase. The exciter then operates as a synchronous generator which delivers the electrical energy required by the rotor of the main electric machine while the main alternator (which is the main electric machine) is operating in generator mode.
The exciter comprises a rotor comprising rotor windings that are connected to the rotor winding of the main electric machine via a rotary rectifier so as to deliver, as output, a DC current for exciting the rotor winding of the main machine.
In order to limit the regulating current, the DC stator winding includes a large number of turns. For example, the startup stator winding includes 480 turns, or 4 ohms, which makes it possible to obtain 1152 ampere-turns of excitation with 2.4 A coming from the regulator. In the startup phase, when the shaft of the turbomachine is not rotating, if the DC stator winding of the exciter is being supplied with DC current, it cannot generate, in the rotor windings of the exciter, an AC current allowing the common shaft coupled to the shaft of the turbomachine to be set in rotation. Specifically, if the DC stator winding of the exciter is being supplied with DC current, there is no current in the rotor winding of the main electric machine and it is not possible to generate a torque for rotating the rotor of the main electric machine. The exciter cannot operate as a synchronous generator in the absence of rotation. It is necessary for the exciter to be supplied with AC electric current for the exciter to develop, across its rotor windings, an AC voltage which, after rectification, supplies the rotor winding of the main electric machine with power. During standstill, the exciter therefore behaves as a transformer having an air gap. If it is chosen to use the DC stator winding of the exciter in the startup phase, the impedance caused by the large number of turns and the high power supply frequency necessitates a supply voltage of 5500 volts in order to provide the 2900 ampere-turns of magnetizing induction. Since this level of voltage is generally not available on the aircraft, it is necessary for it to be generated by means of a voltage step-up converter, the cost and weight of which are prohibitive. It is for this reason that the AC stator winding is provided in the exciter. This winding is intended to be supplied with single-phase AC current during the startup phase so as to induce currents in the rotor of the exciter, which currents will be rectified in order to supply power to the rotor windings of the main electric machine which will then be able to deliver mechanical power allowing the turbomachine to start up.
Ideally, the DC and AC stator windings are in quadrature so as to limit the effect of mutual induction between these two windings and thus to avoid a voltage being induced across the terminals of the generator winding when the startup winding is being supplied with AC current. This quadrature arrangement is described in the French patent application published under the publication number FR 2 348 594.
However, the generation of a substantial voltage across the terminals of the generator winding, which amounts to 900 V in our example, is still observed in the startup phase. This high voltage is due to multiple factors including magnetic leakages and the recovery of the rotary diodes of the bridge rectifier. This voltage may result in damage to the alternator regulator GCU.
In order to avoid this overvoltage, a first solution consists in limiting the ratio of the number of turns between the DC and AC stator windings and in designing an alternator regulator that is capable of withstanding the voltage generated by this ratio in the startup phase. However, this solution has the drawback of decreasing the excitation gain, i.e. the ratio of the current injected into the exciter to the current sent to the rotor of the main electric machine.
A second solution consists in inserting a contactor allowing each of the two DC current lines connecting the DC stator winding to the first regulator to be opened and closed. One of these two lines connects one of the terminals of the DC stator winding to the regulator and the other line connects the other terminal of the DC stator winding to the regulator. This contactor is controlled so as to open each of the two lines in order to isolate the alternator regulator GCU of the rotary machine during the startup phase and to close each of these two lines during the generation phase. This contactor allows the regulator to be protected from over voltages. The main drawback is that this contactor is bulky and expensive since it must both withstand high voltages in the startup phase and large currents in the generation phase.
In both cases the number of turns is limiting since the voltage generated between the two lines in the generation phase may reach the insulating limits of the wires and other insulators. Not being able to increase the number of turns of the AC stator winding means that it is not possible to decrease the excitation current in the AC stator winding in the startup phase, thereby making it necessary to size the two electrical lines and the alternator regulator to withstand large currents.